Serial samples, spanning an observation period of 4 to 10 years, from five patients with anti-Hu associated paraneoplastic neurological syndromes (PNS) were investigated with an enzyme linked immunosorbent assay (ELISA) employing recombinant HuD protein as antigen. In one patient the anti-Hu antibody concentration converted from negative to highly positive levels after the onset of neurological symptoms. These findings argue in favour of the idea that an autoimmune process, which is generated at the beginning of the neurological disease, causes the anti-Hu associated PNS. Serum samples obtained shortly after the beginning of the PNS were available from two other patients. The anti-Hu antibody levels in these latter patients increased from modest to highly positive anti-Hu antibody in follow up samples. In two patients a clear decrease of the anti-HuD antibody concentration together with an improvement of paraneoplastic neurological symptoms after successful tumour treatment were seen. Overall these data suggest that there might be a correlation between the anti-HuD antibody level and the clinical course of paraneoplastic neurological symptoms which supports further the hypothesis that an autoimmune process is responsible for the anti-Hu associated paraneoplastic neurological symptoms.